1. Field of the Invention
The present invention relates to a method of fabricating an optical integrated circuit and, more particularly, to a ridge type semiconductor laser of laterally-coupled distributed feedback (LC-DFB) and a method of manufacturing the same.
2. Description of the Related Art
Distributed feedback (DFB) semiconductor lasers are known as devices which can be used in the fields of optical communication systems such as optical CATVs, pumping light sources for SHG short-wave lasers for high-density information recording or small solid-state lasers, and optical measurement. The DFB semiconductor laser is used for a light source together with the other semiconductor devices.
Conventional distributed feedback semiconductor lasers are probably formed by using two or more steps of epitaxial growth. In a ridge type DFB semiconductor laser formed using two or more steps of epitaxial growth, a grating (diffraction grating) is provided in a laser waveguide layer and thereafter another layer is formed on the waveguide on an epitaxial growth basis.
Recently, in order to avoid the complicated epitaxial growth in two or more steps, the so-called single-growth distributed feedback semiconductor lasers have been developed which are fabricated using one single step of epitaxial growth, i.e., which does not involve any second epitaxial growth.
For example, in R. D. Martin et al. xe2x80x9cCW Performance of an InGaAs-GaAs-AlGaAs Laterally-Coupled Distributed Feedback (LC-DFB) Ridge Laser Diodexe2x80x9d IEEE Photonics Technology Letters, Vol. 7, No. 3, pp 244-246, March 1995, an InGaAs-GaAs-AlGaAs distributed feedback semiconductor laser is disclosed in which an active layer and a cladding layer are formed on a substrate by means of epitaxial growth; a ridge stripe is formed on the cladding layer; and a grating is provided on the top portion of the ridge stripe and on flat portions on both sides thereof. In methods of manufacturing such a laterally-coupled distributed feedback semiconductor laser, the grating is formed on the entire region of the substrate including the top portion of the ridge type waveguide by means of direct writing with electron beams (EB). In a DFB semiconductor laser, in general, a periodic structure is formed which undergoes a variation in shape having a period xcex9 in tile direction in which the laser beam propagates. This results in a periodic variation of the index of refraction which in turn results in an increase in the reflectivity at a wavelength for which the phases of periodically reflected beams match (Bragg reflection), thereby causing laser oscillation. Therefore, the oscillation frequency of a distributed feedback semiconductor laser is determined by the period xcex9 of the periodic structure and, in general, a single longitudinal mode is obtained if the equation xcex9=mxcex/2n is satisfied where m represents an integral number; xcex represents the oscillation wavelength (in vacuum); and n represents the index of refraction of the laser medium. While oscillation generally occurs in the vicinity of wavelength of the Bragg reflection, the period A is determined considering the refractive indices, thicknesses, and aspect ratios of the In1xe2x88x92xGaxAs1xe2x88x92yPy active layer and the cladding layer of the InP ridge stripe material, the reflectivity of the resonator (cleavage planes), and even the lateral optical coupling coefficient.
In the fabrication of such a DFB semiconductor laser, a longitudinally extending ridge stripe is formed by an etching process on the cladding layer and then the grating each line crossing the ridge stripe is formed on and around the ridge stripe. The dry etching process is almost employed for the method for forming the grating because it is excellent in controllability.
However, there are many failures in making the grating with a preferable shape on the side wall of the ridge stripe since it is difficult to make a mask proof against damage from the dry etching and to transfer the mask onto the side wall. The difficulty to fabricate the grating is a serious matter since the property of the DFB semiconductor laser is subject to the shape or status of each line in the grating at the foot of the ridge stripe. The dry etching process may also damage the surface of the substrate and thereby our apprehensions for inferiority of the property of the DFB semiconductor laser remains. On the other hand, the wet etching process hardly damages the surface of the substrate of semiconductor, but it is inferior in controllability because the property of the wet etching is apt to be subject to the crystal plane of semiconductor. The individual use of the wet etching is not suitable for the fabrication of the grating at the foot of the ridge stripe in the ridge type LC-DFB semiconductor laser.
Accordingly, it is very difficult to obtain a secure optical coupling between the grating and the guided light in the ridge stripe waveguide at the foot thereof in the conventional ridge type LC-DFB semiconductor laser.
The present invention confronts the above-described problem, and it is an object of the present invention to provide a laterally-coupled DFB semiconductor laser and a method of manufacturing the same, having a secure optical coupling between the grating and the guided light in the ridge stripe waveguide at the foot thereof.
The object is achieved by a method of manufacturing a ridge type semiconductor laser of laterally-coupled distributed feedback having an active layer made of semiconductor; a cladding layer formed on said active layer; a ridge stripe formed to protrude from said cladding layer; and a grating having a periodic structure in the direction in which the ridge stripe extends and formed on the side walls of the ridge stripe and on flat portions on both sides thereof. This method according to the invention comprises the steps of:
forming a stripe mask having a predetermined width on a cladding layer made of a material for a ridge stripe formed on an active layer made of semiconductor formed on a laser substrate, to form two lateral flat portions from said cladding layer, by a selective wet etching, so as to form a ridge stripe protruding therefrom and having a flat top portion at which the stripe mask capped;
forming a grating mask on said two lateral flat portions, side walls of said ridge stripe and said stripe mask, said grating mask having a periodic structure in the direction in which the ridge stripe extends; and
dry-etching through said grating mask said two lateral flat portions and said side walls of the ridge stripe and then wet-etching said two lateral flat portions and said side walls of the ridge stripe to form a grating made of said material for the ridge stripe on said two lateral flat portions, said side walls of the ridge stripe and said active layer, so as to define a bracket grating portion adjacent to the ridge stripe.
By the present invention, in the step of forming the ridge stripe, the ridge stripe is formed by the wet-etching so that its longitudinal axis of the ridge structure extends parallel to a crystal orientation or direction e.g., a  less than 0{overscore (1)}1 greater than -direction in crystal of the laser substrate. As a result, the ridge stripe have slopes at the side walls each gently down at an angle of 0 to 55xc2x0. This slope angle is gentler than a slope angle about 9020  obtained by only the dry-etching in comparison with that of the conventional ridge stripe. This gentle slope surface at the side wall relaxes the conditions for forming the grating mask, particularly a resist pattern film for the direct electron beam (EB) writing. Therefore, the transferring of pattern may be performed securely in the next step of forming the grating. Moreover, by the dry-etching, the gentle slope side walls of the ridge and both the flat portions of the first cladding layer are deeply dug as valleys between lines of the grading and after that, the wet-etching is performed, so that vertical side walls of crystal face appear each standing up in about vertical angle which define the side wall of the ridge stripe. In this case, each side wall of protruding lines of the grating having (0{overscore (11)}) appears which is a vertical side walls of crystal face and at the same time the damaged portions caused by the dry-etching are removed at the other portions of the grating because of its high etching rate. As a result, a deep grating free from damage is formed as an ideal grating adjacent to an emitting region in the active layer in the laser device.
In a first aspect of the invention of the method of manufacturing a semiconductor laser, the method further comprises a step of forming a contact layer on the cladding layer to be connected to an electrode, wherein the contact layer disposed under the stripe mask is used for a second mask proofing against the selective wet etching in the step of forming the ridge stripe.
In a second aspect of the invention of the method of manufacturing a semiconductor laser, the method further comprises a step of forming a wet-etching termination layer for terminating a wet-etching within the cladding layer so that the wet-etching termination layer is sandwiched between a pair of cladding layers. This interposing of the wet-etching termination between the cladding layers may facilitate the grating formation without exposing of the active layer of semiconductor and the guiding layer being apt to have occurrence of non-emitting recombining portions at the exposed surface.
In a third aspect of the invention of the method of manufacturing a semiconductor laser, the wet-etching termination layer is made of InGaAsP or InGaAs.
In a fourth aspect of the invention of the method of manufacturing a semiconductor laser, said active layer is a bulk layer, a single quantum well layer, or a multiple quantum well layer mainly composed of In1xe2x88x92xGaxAs1xe2x88x92yPy (where 0xe2x89xa6x less than 1, 0xe2x89xa6yxe2x89xa61); and said cladding layer is made of InP.
In a fifth aspect of the invention of the method of manufacturing a semiconductor laser, said contact layer is made of InGaAsP or InGaAs.
In a sixth aspect of the invention of the method of manufacturing a semiconductor laser, said selective wet etching for said cladding layer of InP is performed using a hydrochloric acid type etchant.
In a seventh aspect of the invention of the method of manufacturing a semiconductor laser, said step of forming a grating mask includes;
a step of forming a protective film and a resist layer in this order to cover on said two-lateral flat portions, the side walls of said ridge stripe and said stripe mask; and
a step of forming a latent image of a grating mask having a periodic structure in the direction in which the ridge stripe extends on said resist layer and then forming a grating mask by developing said resist layer.
In an eighth aspect of the invention of the method of manufacturing a semiconductor laser, said step of forming the latent image includes the step of forming said latent image using a direct electron beam writing process.
In a ninth aspect of the invention, the method of manufacturing a semiconductor laser further comprises;
a step of removing said protective film and said stripe mask after the forming step of said grating;
a step of forming to form an insulator layer on an entire surface of said grating and said flat top portion; and
performing etching to expose only said flat top portion on said insulator layer and, thereafter, forming an electrode on said flat top portion.
Furthermore, a ridge type semiconductor laser of laterally-coupled distributed feedback according to the invention comprises;
an active layer made of semiconductor;
a cladding layer formed on said active layer;
a ridge stripe formed to protrude from said cladding layer; and
a grating having a periodic structure in the direction in which the ridge stripe extends and formed on side walls of the ridge stripe and on flat portions on both sides thereof, wherein said grating has bracket grating portions defined adjacent to the ridge stripe and each having a slope surface extending from a flat top portion of the ridge stripe to a top face the grating and coupling the side walls of the ridge stripe and the grating.
In a first aspect of a ridge type semiconductor laser of laterally-coupled distributed feedback according to the invention, said active layer is a bulk layer, a single quantum well layer, or a multiple quantum well layer mainly composed of In1xe2x88x92xGaxAs1xe2x88x92yPy (where 0xe2x89xa6x less than 1, 0xe2x89xa6yxe2x89xa61); and said cladding layer is made of InP.
In a second aspect of a ridge type semiconductor laser of laterally-coupled distributed feedback according to the invention, the laser further comprises a wet-etching termination layer for terminating a wet-etching within the cladding layer so as to be sandwiched between a pair of cladding layers.
In a third aspect of a ridge type semiconductor laser of laterally-coupled distributed feedback according to the invention, the wet-etching termination layer is made of InGaAsP or InGaAs.
According to the present invention, the grating can be uniformly even at the step portions between the ridge stripe and the flat portions. Specifically, the present invention takes advantageous use of the selective wet-etching and the dry-etching thereafter to achieve the fabrication of the bracket grating portions for coupling the side walls of the ridge stripe and the grating and each having a slope side wall surface, thereby making it possible to manufacture laterally-coupled DFB ridge semiconductor lasers with high yield, at low cost, and with a high quality and secure optical laterally-coupling.